Surface passivation by quantum exclusion using multiple layers

ABSTRACT

A semiconductor device has a multilayer doping to provide improved passivation by quantum exclusion. The multilayer doping includes a plurality M of doped layers, where M is an integer greater than 1. The dopant sheet densities in the M doped layers need not be the same, but in principle can be selected to be the same sheet densities or to be different sheet densities. M−1 interleaved layers provided between the M doped layers are not deliberately doped (also referred to as “undoped layers”). Structures with M=2, M=3 and M=4 have been demonstrated and exhibit improved passivation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. provisionalpatent application Ser. No. 61/355,049, filed Jun. 15, 2010, whichapplication is incorporated herein by reference in its entirety. Thisapplication is also related to U.S. patent application Ser. No.12/965,790, filed Dec. 12, 2010, which is assigned to the same assigneeas the present application.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The invention relates to semiconductor devices in general andparticularly to silicon devices that rely on surface passivation fortheir operation.

BACKGROUND OF THE INVENTION Statement of the Problem

Surface Passivation, Quantum Efficiency, and Stability ofBack-Illuminated Imaging Detectors

Surfaces and interfaces have long been known to be critical to theperformance of virtually all solid-state devices, and imaging devices inparticular. Surface passivation technologies were critical to both theinvention of the transistor and to the development of reliable processesfor planar integrated circuits, which launched the semiconductorrevolution. The revolution in solid-state imaging devices began in 1969,with the invention of charge-coupled devices (CCDs). Surfaces andinterfaces posed problems from the beginning, and many of the laterimprovements in CCD design were directed toward achieving control overthe quality of interfaces near the device's front surface. TexasInstruments demonstrated the first back-illuminated CCDs as early as1974.

NASA quickly realized the potential for solid-state imaging devices forastronomical imaging in space, and began developing CCDs and cameras forspace instruments, including the Hubble Space Telescope. The JetPropulsion Laboratory (JPL) played a key role in this development. JPLwas responsible for developing the Wide Field/Planetary Camera (WF/PC),an important instrument for the Hubble Space Telescope (HST) which wouldlater produce the iconic images associated with NASA and the HST. One ofthe most important science requirements for WF/PC detectors was theachievement of high quantum efficiency (QE) over a wide spectral rangewith photometric stability better than 1%. In particular, the HSTdetectors were required to detect UV light down to the Lyman-α line ofatomic hydrogen, situated in the far ultraviolet region of the spectrum,at a wavelength of 121.6 nm. As evidenced by the history of WF/PC II,between quantum efficiency and stability, stability is the moreimportant detector performance specification.

In order to meet these requirements, back illumination was consideredessential, because absorption in the front-surface gate electronics ofCCDs rendered conventional, front-illuminated CCDs virtually blind inthe ultraviolet. Unfortunately, back illumination led to instabilitiesin the response, as the substrate removal process necessary to exposethe light-sensitive volume of the detector was found to create anunstable back surface of the CCD. Because of the low doping levels andhigh density of unpassivated defects in the surface, changes in theenvironment affected the response of thinned detectors. In particular,the back surface potential at the Si—SiO₂ interface, which is criticalfor high efficiency collection of photogenerated charge, depends on boththe physical environment and the illumination history of the device.Early efforts to control the back surface potential were based onoptimizing the thinning process to leave a thin p+ layer on the backsurface of the CCD. This approach proved inadequate, as poor uniformityof thinning, low surface dopant concentrations, and lack of control overthe dopant profile presented insurmountable barriers to achieving therequired stability. This problem came to a head when the WF/PCinstrument was undergoing thermal-vacuum testing in advance of theoriginally planned December 1984 launch date. The WF/PC detectorsexhibited quantum efficiency hysteresis (QEH) over an order of magnitudeworse than the 1% stability specification set by HST's sciencerequirements. To better address this problem for HST and futureinstruments, JPL began a concerted effort to solve the back-surfacepassivation problem, which would encompass the development of a UV-floodprocess, the deposition of high work function metals to act as Schottkybarriers, and the use of a biased back-surface contact. While none ofthese approaches succeeded in time for WF/PC (launched in 1990) andWF/PC II (launched in 1992), these technologies evolved into the modernstate-of-the-art technologies of chemisorption passivation (Lesser etal.) and shallow ion-implantation followed by a laser anneal.Nevertheless, even in their modern incarnations, state-of-the-artsurface passivation technologies have not solved all of the problemsraised by HST detector development in the 1980's.

A discussion of some of the prior art methods is given hereinbelow. Inparticular, one of the best methods of passivating surfaces in silicondevices known in the prior art is referred to as delta doping.

Known in the prior art is Hoenk et al., U.S. Pat. No. 5,376,810, issuedDec. 27, 1994, which is said to disclose a backside surface potentialwell of a backside-illuminated CCD that is confined to within about halfa nanometer of the surface by using molecular beam epitaxy (MBE) to growa delta-doped silicon layer on the back surface. Delta-doping in an MBEprocess is achieved by temporarily interrupting the evaporated siliconsource during MBE growth without interrupting the evaporated p+ dopantsource (e.g., boron). This produces an extremely sharp dopant profile inwhich the dopant is confined to only a few atomic layers, creating anelectric field high enough to confine the backside surface potentialwell to within half a nanometer of the surface. Because the probabilityof UV-generated electrons being trapped by such a narrow potential wellis low, the internal quantum efficiency of the CCD is nearly 100%throughout the UV wavelength range. Furthermore, the quantum efficiencyis quite stable.

Also known in the prior art is Cunningham et al., U.S. Pat. No.6,107,619, issued Aug. 22, 2000, and Cunningham et al., U.S. Pat. No.6,346,700, issued Feb. 12, 2002, both of which are said to disclose adelta-doped hybrid advanced detector (HAD) is provided which combines atleast four types of technologies to create a detector for energeticparticles ranging in energy from hundreds of electron volts (eV) tobeyond several million eV. The detector is sensitive to photons fromvisible light to X-rays. The detector is highly energy-sensitive fromapproximately 10 keV down to hundreds of eV. The detector operates withmilliwatt power dissipation, and allows non-sequential readout of thearray, enabling various advanced readout schemes.

Also known in the prior art is Nikzad et al., U.S. Pat. No. 7,786,421,issued Aug. 31, 2010, which is said to disclose a system and method formaking solid-state curved focal plane arrays from standard andhigh-purity devices that may be matched to a given optical system. Thereare two ways to make a curved focal plane arrays starting with the fullyfabricated device. One way, is to thin the device and conform it to acurvature. A second way, is to back-illuminate a thick device withoutmaking a thinned membrane. The thick device is a special class ofdevices; for example devices fabricated with high purity silicon. Onesurface of the device (the non VLSI fabricated surface, also referred toas the back surface) can be polished to form a curved surface.

Also known in the prior art is Blacksberg et al., U.S. Pat. No.7,800,040, issued Sep. 21, 2010, which is said to disclose a method forgrowing a back surface contact on an imaging detector used inconjunction with back illumination. In operation, an imaging detector isprovided. Additionally, a back surface contact (e.g. a delta-dopedlayer, etc.) is grown on the imaging detector utilizing a process thatis performed at a temperature less than 450 degrees Celsius.

There is a need for systems and methods that provide improvedpassivation of semiconductor devices.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a silicon device,comprising a silicon wafer bounded by a first surface and a secondopposite the first surface, the silicon wafer having a device fabricatedon one of the first surface and the second surface; the silicon waferhaving a doping profile situated adjacent at least one of the firstsurface and the second surface, the doping profile having a plurality Mof doped layers; each of plurality M of doped layers having a thicknessof less than 10 Angstroms, and a dopant sheet density at least 10¹⁴cm⁻², where M is an integer greater than 1; the plurality M of dopedlayers separated from each other by M−1 interleaved layers of silicon,at least one of the M−1 interleaved layers of silicon having a thicknessin the range of 10 Angstroms to 30 Angstroms; the silicon wafer havingat least one of the first surface and the second surface electronicallypassivated irrespective of a density of defects present on therespective one of first surface and the second surface.

In one embodiment, M is at least 3, and the plurality M of doped layersare separated by M−1 interleaved layers of silicon, at least two of theM−1 interleaved layers of silicon having substantially equalthicknesses.

In another embodiment, M is at least 3, and the plurality M of dopedlayers are separated by M−1 interleaved layers of silicon, at least twoof the M−1 interleaved layers of silicon having unequal thicknesses.

In yet another embodiment, at least one of the M−1 interleaved layers ofsilicon has a dopant sheet density of less than 10¹³ cm⁻².

In still a further embodiment, a dopant gradient of at least one decadeper nm exists between one of the plurality M of doped layers and anadjacent one of the M−1 interleaved layers of silicon.

According to another aspect, the invention relates to a silicon device,comprising a silicon wafer bounded by a first surface and a secondopposite the first surface, the silicon wafer having a device fabricatedon one of the first surface and the second surface; the silicon waferhaving a doping profile situated adjacent at least one of the firstsurface and the second surface, the doping profile having a plurality Mof doped layers; each of plurality M of doped layers having a thicknessof less than 40 Angstroms, a dopant sheet density at least 10¹⁴ cm⁻² anda dopant gradient of at least one decade per nm, where M is an integergreater than 1; the silicon wafer having at least one of the firstsurface and the second surface electronically passivated irrespective ofa density of defects present on the respective one of first surface andthe second surface.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a diagram that illustrates calculated electric fieldscomparing delta-doped surfaces to other methods of surface doping in theprior art.

FIG. 2 is a diagram that illustrates calculated electric potentialscomparing delta-doped surfaces to other methods of surface doping in theprior art.

FIG. 3 is a diagram that illustrates near surface electric field. AllMBE layers represented in the figure contain a surface dipole region anda silicon bulk region, where electric fields are very high. Multilayer(also referred to as “multiple layer’”) doping creates a third region inthe surface which is absent in the delta-doped surface. In this region,the electric fields are also very high, but the average field isrelatively low. Curve 310 represents calculated data for a single deltadoped layer, curve 320 represents calculated data for a multilayerhaving two doped layers, and curve 330 represents calculated data for amultilayer having four doped layers.

FIG. 4 is a diagram that illustrates electronic potential comparingdelta-doping and multilayer doping. The surface dipole and silicon bulkpotentials are very similar in delta-doped and multilayer dopedsurfaces. Multilayer doping creates a wider potential barrier separatingthe surface from the bulk regions, which creates greater isolation ofsurface from bulk and greatly increases the surface conductivity. Curve410 represents calculated data for a single delta doped layer, curve 420represents calculated data for a multilayer having two doped layers, andcurve 430 represents calculated data for a multilayer having four dopedlayers.

FIG. 5 is a diagram that illustrates the electron states near a surfacethat is passivated by delta-doping. This plot shows the quantizedelectron states calculated for the L, X, and Gamma conduction bands.Each state is represented by the probability density as a function ofdepth, shifted and scaled so that the vertical position reflects theenergy of the state. The conduction band edge for the longitudinal Lband is also plotted for comparison. Curve 510 represents calculateddata for a single delta doped layer, curve 520 represents calculateddata for a multilayer having two doped layers, and curve 530 representscalculated data for a multilayer having four doped layers.

FIG. 6 is a diagram that illustrates the electron states near a surfacepassivated by a multilayer having two doped layers. In comparison withdelta-doped surface (FIG. 5), the increased barrier height provided bymultilayer doping results in improved isolation of bulk silicon from thesurface, and also creates a few surface resonances (shown in bold).These are electron states that have locally enhanced probabilitydensities near the Si—SiO₂ interface. Curve 610 represents calculateddata for a single delta doped layer, curve 620 represents calculateddata for a multilayer having two doped layers, and curve 630 representscalculated data for a multilayer having four doped layers.

FIG. 7 is a diagram that illustrates the hole density near surfacespassivated by delta-doping and multilayer doping. Consistent with thesurface conductivity measurements, multilayer doping dramaticallyincreases the concentration of majority carriers near the surface. Curve710 represents calculated data for a single delta doped layer, curve 720represents calculated data for a multilayer having two doped layers, andcurve 730 represents calculated data for a multilayer having four dopedlayers.

FIG. 8 is a diagram that illustrates the electron states near a surfacepassivated by a multilayer having four doped layers. In comparison withdelta-doped surface (FIG. 5) and the multilayer having two doped layers(FIG. 6), increasing the number of doped layers in the multilayerfurther improves the isolation of bulk silicon from the surface, andalso enhances the localization of the few surface resonances (shown inbold). The lowest energy surface resonances may be consideredquasi-bound. In FIG. 8, the near surface hole density for the heavy holeband is calculated based on trapping of holes at the surface. Holetrapping significantly reduces the density of holes for the delta-dopedsurface, the potential barrier between surface and detector for thedelta-doped surface, but has a relatively minor effect on the holedensity created by multilayer doping. Curve 810 represents calculateddata for a single delta doped layer, curve 820 represents calculateddata for a multilayer having two doped layers, and curve 830 representscalculated data for a multilayer having four doped layers.

FIG. 9 is a diagram that illustrates delta-doping with surfacedeactivation: Quantized electron states show reduced tunneling barrierseparating surface from bulk; however, there are no surface confinedstates that could trap hot carriers.

FIG. 10 is a diagram that illustrates delta-doping with surface trappingof holes: Quantized electron states show a strong tunnel barrierseparating surface from bulk; a deep surface well produces a smallnumber of surface confined states that could trap hot carriers, but manymore unconfined states.

FIG. 11 is a diagram that illustrates a two layer multilayer withsurface deactivation. In comparison with delta-doped surface (FIG. 9),the increased barrier height provided by multilayer doping results inimproved isolation of bulk silicon from the surface, and also creates afew surface resonances (shown in bold).

FIG. 12 is a diagram that illustrates a two layer multilayer withsurface trapping of holes. In comparison with a delta-doped surface(FIG. 10), the two layer multilayer provides a stronger tunnel barrierisolating surface from bulk; however, in this case, the main advantageof multilayer doping lies in the two order of magnitude increase in holesheet density near the surface.

FIG. 13 is a diagram that illustrates electron states near a surfacepassivated by a multilayer having four doped layers. In comparison withdelta-doped surface (FIG. 9) and the multilayer having two doped layers(FIG. 11), increasing the number of doped layers in the multilayerfurther improves the isolation of bulk silicon from the surface, andalso enhances the localization of the few surface resonances (shown inbold). The lowest energy surface resonances may be consideredquasi-bound.

FIG. 14 is a diagram that illustrates a multilayer having two dopedlayers with surface trapping of holes: In comparison with a delta-dopedsurface (FIG. 10), the multilayer having four doped layers provides astronger tunnel barrier isolating surface from bulk; however, in thiscase, the main advantage of multilayer doping lies in the two order ofmagnitude increase in hole sheet density near the surface.

FIG. 15 is a diagram that illustrates delta doping robustness againstdopant deactivation with varying levels of deactivation. The calculationassumes a density of surface traps of 5×10¹² cm⁻².

FIG. 16 is a diagram that illustrates delta doping robustness againstsurface charge with full activation, but varying densities of surfacetraps.

FIG. 17 is a diagram that illustrates the robustness of a multilayerwith two doped layers against dopant deactivation with varying levels ofdeactivation. The calculation assumes a density of surface traps of5×10¹² cm⁻².

FIG. 18 is a diagram that illustrates the robustness of a multilayerwith two doped layers against surface charge with full activation, butvarying densities of surface traps.

FIG. 19 is a diagram that illustrates the robustness of a multilayerwith four doped layers against dopant deactivation with varying levelsof deactivation. The calculation assumes a density of surface traps of5×10¹² cm⁻².

FIG. 20 is a diagram that illustrates the robustness of a multilayerwith four doped layers against surface charge with full activation, butvarying densities of surface traps.

FIG. 21 is a schematic, not to scale, diagram that illustrates a crosssection of a wafer having multilayer doping according to principles ofthe invention.

DETAILED DESCRIPTION Prior Art Surface Passivation Technologies

Chemisorptioin Charging

Chemisorption passivation in its modern form evolved from early effortsat JPL to use high work function metals to form a Schottky barrier onthe back surface of thinned CCDs. A Schottky barrier exists due tocharge transfer across an interface between dissimilar materials. Therationale behind using Platinum to form the Schottky barrier was thatthe high work function of Platinum would create a near surface electricfield of the right polarity in the silicon surface to drivephotogenerated electrons away from the back surface and toward thefront-surface detector electronics (in fact this is a generalrequirement that applies to all of the various surface passivationtechnologies for imaging detectors). In the course of JPL's developmentof sensors for WF/PC II, it was discovered that the surface chargingmechanism responsible for improving the detector quantum efficiency withthe Pt “flash gate” technology was not (as originally intended andexpected) the formation of a Schottky barrier at the surface, butinstead involved the accumulation of negatively charged O₂ ⁻ ions on theoxide surface through a chemisorption process. Thus a similar chargingmechanism underlay both the earlier UV flood process and the Pt “flashgate”; unfortunately, neither of these processes provided adequatestability, nor did subsequent improvements and refinements successfullyresolve the surface passivation problem. A key part of the difficultylay with the oxide. The stability of chemisorbed charge was found to becritically dependent on the quality and thickness of the oxide. Theformation of a high quality thermal oxide required temperatures thatexceeded the tolerance of imaging detectors, so a low temperature “flashoxide” process was developed based on exposing the surface to steam atmoderate temperatures. Unfortunately, the “flash oxide” failed tostabilize the device, as changing environmental conditions (especiallywith respect to exposure to hydrogen) could reverse the polarity of thechemisorbed charge with a catastrophic effect on detector quantumefficiency and spectral response. As a result of these limitations,development of the Pt “flash gate” was abandoned, and was not used indetectors flown on WF/PC II.

Subsequent development efforts at the University of Arizona led toseveral innovations and refinements of the chemisorption process,including the use of thicker, higher quality oxide layers, switching tometals that aren't sensitive to poisoning by hydrogen exposure, andcoating the metal layer with thermally deposited HfO₂ dielectric layerto stabilize the chemisorbed charge against environmental variations.Chemisorption devices have been used in both ground and space-basedobservatories. Despite these advances, surface passivation bychemisorption charging is limited to visible and near ultravioletwavelengths by absorption in the dielectric layers required to hold andstabilize chemisorbed charge. Chemisorption charging is also subject toirreversible damage by ionizing radiation. In particular, chemisorptionis unstable to the ionizing effects of deep ultraviolet light, which iswell known to liberate H+ ions and create traps in SiO₂ and otherdielectric layers used as insulating layers in the semiconductorindustry. Finally, chemisorption creates fixed charge embedded in aninsulating layer, and does not provide a conductive path in the siliconfor lateral transport of photogenerated majority carriers. Therequirement for a conductive back surface has been found to be importantin applications requiring fully-depleted imaging devices and is likelyto be important in applications that require exposure to high intensitylight sources, such as deep ultraviolet lasers.

Ion Implantation and Laser Anneal

Ion implantation is a standard process used in the semiconductorindustry to selectively dope semiconductor surfaces for deviceapplications. The process is based on directing energetic dopant atomstoward a semiconductor surface to implant a desired dose in the crystallattice. The implanted atoms are not located on electrically activelattice sites of the crystal, and the implantation process creates ahigh density of defects that degrade the quality of the semiconductor.Implantation therefore requires a high temperature thermal process toanneal away many of the crystal defects and to electrically “activate”the implanted atoms by allowing a fraction of the atoms to move frominterstitial sites into crystal lattice sites. However, the temperaturerequired for “activation” is incompatible with the thermal constraintsof device processing (note that there are isolated exceptions, in whichrefractory metals are used in the front-surface electronics in order toenable high-temperature furnace anneals of ion-implanted layers;however, refractory metals require specialized processes and imposeconstraints on metal conductivity that are not compatible with allimaging device technologies and applications). Therefore, in order toadapt ion implantation to the requirements and constraints of imagingdetectors, several process modifications are important. First, very lowenergy implantation is desired in order to create shallow dopantprofiles suitable for detection into the UV range. Second, a veryshallow annealing process is implanted using pulsed lasers to heat onlythe near-surface region. Third, as opposed to selective processes usedto dope small areas in transistors and other devices, ion implantationof back-illuminated imaging detectors requires that processes beoptimized to achieve uniform doping over the entire detector surface.This is particularly challenging for laser annealing, which tends tocreate “brick wall” artifacts in imaging detectors.

Ion implantation provides higher dopant concentrations and more controlover the incorporated dopant profile than the diffusion-related profilesthat were originally used in the optimal thinning process for WF/PCdetectors. However, this is only a relative advantage, as the physics ofion implantation and the necessity of maintaining process compatibilitywith imaging detectors provide only limited flexibility in designing theshape, depth, peak position, amplitude, and uniformity of the dopantprofile. These constraints in turn place limits on the ability to useion implantation for optimization of detector performance, especiallywith regard to UV quantum efficiency, defect-related dark currentgeneration, and “deep depletion” for improved spatial resolution. Inparticular, ion implantation cannot create abrupt dopant profiles, which(as we shall see) are extremely important for surface passivation.

The inability of ion implantation/anneal processes to create abruptdopant profiles is a limitation that extends beyond the field of imagingdetector technologies. Achieving abrupt dopant profiles is one of themajor challenges faced by the semiconductor industry in its ongoingefforts to fabricate integrated circuits with higher densities. The goalof creating sharper ion-implanted dopant profiles is the subject of anextensive literature in semiconductor processing journals. One of thelimitations lies with broadening of implanted dopant distributionsduring the annealing process. The phenomenon of transient enhanceddiffusion (TED) refers to the anomalously high diffusion rates observedduring the annealing of implanted dopant distributions. The observedhigh rates of diffusion are related to defects inevitably created by theimplantation process.

Despite these limitations, ion-implanted imaging devices are currentlybeing used in a wide variety of imaging applications, includingscientific imaging detectors deployed in space. State-of-the-artion-implanted devices are at the heart of the Wide-Field Camera 3 (WFC3)instrument, which recently replaced the Wide Field/Planetary Camera 2instrument on the Hubble Space Telescope. However, despite significantadvances in the two decades following the development of WF/PC 2detectors, the state-of-the-art ion-implanted devices in WFC3 stillexhibit quantum efficiency hysteresis (QEH) that is outside the HSTspecifications. Based on extensive characterization of these devices,the observed QEH appears to be related to charge traps in the silicon,which are probably an artifact inherent in the ion implantation/annealprocess used for back surface passivation. The temporary solution thatis currently in use for WFC3 is a periodic exposure of the device tointense light, in order to fill these traps; maintaining the detector ata low operating temperature stabilizes the trapped charge sufficientlyto collect scientific data.

Delta Doping

At the same time that detector development for WF/PC 2 was underway, JPLscientists began developing a unique surface passivation technologybased on the epitaxial growth of highly-doped silicon. Whereasconventional crystal growth technologies require temperatures thatexceed the tolerance of CCDs, JPL had conducted pioneering work in the1980's on a low temperature molecular beam epitaxy process that couldachieve epitaxial growth of silicon at CCD-compatible temperatures(below 450° C.). These efforts led to the development and demonstrationof delta-doped CCDs in 1992, in which low temperature MBE growth wasused to form an ultrathin, delta-doped silicon layer on afully-functional, thinned CCD.

The delta-doping process derives its name from a dopant profile thatresembles the mathematical delta function. Delta-doping achieves anexceptionally abrupt dopant profile by interrupting the flux of siliconatoms, depositing dopant atoms at a density of about one third of amonolayer (approximately 2×10¹⁴ dopant atoms/cm²), and encapsulating thedopant atoms by growing a 1-2 nm silicon “cap” layer. Because thedopants are incorporated in a growth process, MBE-grown layers do notsuffer from the defects created by lattice damage during ionimplantation, nor do they require a high temperature annealing processthat would limit the ability to generate abrupt dopant profiles. Theprocess of interrupting and then restarting the silicon flux duringgrowth concentrates the dopant atoms in a layer that is only a fewatomic layers thick, which can be precisely located within a few atomiclayers of the surface. This precision is the ultimate in abrupt dopingprofiles and cannot be achieved by ion implantation or any otherconventional doping process (e.g., ion implantation and diffusion).

Delta-doping achieves nearly 100% internal quantum efficiency throughthe far and extreme ultraviolet spectral range, with no apparentdegradation in performance from exposure to ionizing radiation, nomeasurable quantum efficiency hysteresis and no apparent sensitivity toenvironmental conditions even after several years of storage at roomtemperature in the presence of oxygen and water vapor. Whereasdelta-doping was initially demonstrated using elemental boron as adopant material for surface passivation of thin, n-channel CCDs,subsequent development efforts have shown that delta-doping worksequally well for surface passivation of back-illuminated photodiodearrays, CMOS imaging arrays, fully-depleted p-channel CCDs (requiringn-type delta-doping using antimony as the dopant material), andelectron-multiplied CCDs (which use a high-gain output register forphoton-counting applications).

MBE Doping Using a Uniform Dopant Profile

MIT Lincoln Labs (MIT-LL) has recently developed a surface passivationprocess that uses an MBE-grown silicon layer that is 5 nm in thicknessand contains a uniform distribution of boron (B) at a concentration of2×10²⁰ cm⁻³ (corresponding to a sheet density of 1×10¹⁴ cm⁻²). MIT-LLclaims the achievement of near 100% internal quantum efficiency and nomeasurable hysteresis. Studies done by MIT-LL on exposure ofback-illuminated CCDs to extreme ultraviolet radiation prove that theMBE-grown layer is more radiation hard than either chemisorptioncharging or ion implantation. The improved hardness to radiation ofMBE-passivated devices is attributed to the total amount of chargeincorporated into the passivation layers and the relative thickness ofthe oxide layers on the surfaces. Citing greater mobility of electronsthan holes injected into oxides by ionizing radiation, the authors ofthe study assert that exposure to radiation produces positive charge inthe oxide layer that compensates dopants in the surface passivationlayer. Of the devices compared in this study (which did not include adelta-doped device), the uniform, MBE-grown passivation layer containsthe greatest charge density (1×10¹⁴ cm⁻²), and the thinnest oxide (1-2nm). Compared to uniformly doped layers grown by MIT-LL, JPL'sdelta-doping process achieves higher charge densities with thinnerMBE-grown layers, and the multilayer passivation layer described hereachieves even higher charge densities.

In a related study, researchers at MIT-LL found that the MBE-grown layeris responsible for excess surface-generated dark current. They were ableto mitigate this problem with a 400° C. anneal in hydrogen, whichpassivates surface states in the native oxide and thereby reduces thesurface-generated dark current. Even with the incorporation of hydrogenpassivation, the observed dark current is still an order of magnitudehigher than an equivalent front-illuminated device, indicating thatback-surface defects are not fully passivated. JPL's delta-dopingprocess does not require hydrogen passivation to achieve low darkcurrent.

Limitations of Delta-Doping and the Need for a New Technology

FIGS. 1 and 2 compare delta-doping with other surface dopingtechnologies according to calculated near-surface electric fields andelectronic potentials. The slowly varying dopant profiles created bydiffusion and ion implantation produce weak, slowly varying electricfields and potentials, which provides poor isolation of surface frombulk and leads to instabilities in the response (as seen in the WFC3detector performance data from the Hubble Space Telescope). Incomparison, the plots show that delta-doping creates the strongestelectric fields and the highest energy barriers of any surfacepassivation technology in the prior art. This is consistent with quantumefficiency and stability measurements that show how effective apassivation layer delta-doping provides. Nevertheless, recentmeasurements suggest that the surface density of mobile holes is twoorders of magnitude lower than the surface density of dopant atoms inthe delta-doped layer. This discrepancy is significant, both for itsconsequences for device performance, and for its implication that animproved surface passivation technology is necessary. The inventivetechnology addresses this need, as described below.

FIG. 1 is a diagram that illustrates calculated electric fieldscomparing delta-doped surfaces to other methods of surface doping in theprior art.

FIG. 2 is a diagram that illustrates calculated electric potentialscomparing delta-doped surfaces to other methods of surface doping in theprior art.

State-of-the-art passivation technologies in the prior art: These twoplots present results of calculations that were done to comparedelta-doping with other surface doping technologies in the prior art.The plots show that delta-doping creates the strongest electric fieldand the highest energy barrier of any prior art. The key to achievingthis is the creation of abrupt dopant profiles by MBE. Whereas theprinciples illustrated by these models are correct, recent resultssuggest the existence of chemical and/or physical mechanisms that causethe behavior of real delta-doped surfaces to deviate from the models.

Surface Passivation by Quantum Exclusion

Multilayer Doping: Introduction and General Description

Various methods of surface passivation technologies are well known inthe art that predate the technologies specifically developed forback-illuminated detectors (as well as solar cells, which have verysimilar requirements as detectors). These well-known technologiesinclude the growth of thermal oxides, annealing in hydrogen, and thegrowth or deposition of high performance insulators (e.g., high-k oxidesthat are the subject of a large literature in semiconductor technology).These technologies are directed toward eliminating or mitigating theinfluence of electrically active defects, as opposed to charging thesurface to create favorable fields and potentials. With the exception ofdelta-doping, all of the surface charging methods described in theprevious section also rely on such methods—especially hydrogenpassivation—to help improve stability and efficiency of surfacepassivation. This reliance on low defect densities presents a problemfor stability, because ionizing radiation—including exposure to highenergy photons (e.g., deep ultraviolet, far ultraviolet, and extremeultraviolet light, all of which are technologically and scientificallyimportant). One of the advantages of the inventive technology is theimprovement of stability irrespective of surface defects.

Stability is an important performance metric, as charging anddischarging of surfaces and interfaces can play havoc with devices. Asillustrated by the history of detector development for the Hubble SpaceTelescope, back-illuminated optical detectors require surfacepassivation in order to achieve high quantum efficiency, low darkcurrent and stable response. Passivation requires a process to create apassivation layer which is thin enough to be transparent at all detectedwavelengths. For optimal efficiency, the passivation layer must create astrong electric field in the silicon near the detector surface in orderto prevent minority carriers from recombining or becoming trapped at thesurface. To suppress surface-generated dark current, the passivationprocess must either eliminate surface states or suppress the injectionof thermally-generated charge from the surface into the bulk silicon. Inorder to mitigate quantum efficiency hysteresis, the fields created bythe passivation layer must be stable against perturbations of thesurface potential, which may be caused by trapping and detrapping ofelectrons and holes at the surface. Surface passivation technologies arealso distinguished by robustness, or the ability to reduce or delaydegradation of detector performance in a harsh environment (e.g.,mitigating or preventing permanent changes to the detector performancethat may accompany chemical or physical damage to the surface due tocontaminants and/or ionizing radiation). Recent data from the Wide FieldCamera 3 instrument on the Hubble Space Telescope, as well the need forimproved lifetime in DUV, FUV and EUV detectors, demonstrates that thereis a need for surface passivation technologies with improved stabilityand robustness.

The inventive technology achieves improved stability and robustnesscompared to the prior art by using multilayer doping to achieveexceptionally high density of dopant in a thin passivation layer, thusisolating the detector from the surface irrespective of the density ofsurface defects. The design and implementation of the inventivetechnology requires nanometer-scale control over the semiconductorcomposition. On this length scale, electron and hole interactions withthe surface are governed by quantum mechanics, and the isolation ofsurface from bulk is achieved through control of the quantum behavior ofelectrons and holes—hence the terminology, surface passivation byquantum exclusion.

The multilayer doping technology improves the performance of solid-statedetectors compared to the prior art in the following ways:

It provides a tunneling barrier that suppresses the generation andtransport of minority carriers from surface to bulk (thereby improvingstability and improving signal-to-noise performance by reducing thesensitivity to surface states).

It provides a tunneling barrier that suppresses the transport oflow-energy (“thermal”) minority carriers from the bulk silicon to thesurface, and reduces the probability of such carriers from interactingwith traps at the surface (enabling high quantum efficiency andimproving stability).

It minimizes the probability of trapping or recombination of high-energy(“hot”) minority carriers either at the surface or within thepassivation layer, and (conversely) promotes the transport of suchcarriers away from the surface and into the bulk silicon (enabling highquantum efficiency and improving stability and robustness).

It provides high surface conductivity in order to facilitate lateraltransport of excess majority carriers, thus mitigating localaccumulation of majority carriers and helping to maintain the detectorsurface at a constant potential under all illumination conditions(improving stability by mitigating dynamic charging of surface states).

It isolates the fields and potentials in the bulk silicon from beinginfluenced by temporary or permanent changes in the surface potential,thus mitigating any effects on detector performance caused by chemicaland physical changes to the surface and oxide/antireflection-coating(e.g., due to radiation damage, hot carrier injection, or other damagingeffects of the environment).

The principles, methods, and structures for achieving surfacepassivation by quantum exclusion using multilayer doping are describedbelow.

Multilayer doping interposes a thin crystal between the surface andsilicon detector that is transparent to high-energy (“hot”) carriers andopaque to low energy (thermal) carriers. Effectively, multilayer dopingcreates an electronic surface that is isolated from and independent ofthe physical surface. The layer itself is designed according to theprinciples of quantum mechanics to isolate and decouple surfacestates/defects from minority carrier states in the detector(semiconductor “bulk”), while minimizing the probability that hotcarriers will be captured within the layer or at the surface. Asdescribed above, this quantum mechanical decoupling of the electronicand physical surfaces (quantum exclusion) is extremely important inimaging detectors, because of the requirement to prevent environmentalconditions (chemical or physical changes to surface coatings, adsorbedor chemisorbed molecules, and external fields) from affecting theperformance of the imaging detector (especially sensitivity, stability,and noise).

Whereas the context of this invention is the field of imaging detectors,virtually all semiconductor devices are affected by defects in surfacesand interfaces. It is stipulated that the concept of surface passivationby quantum exclusion is more general both in method and application;that other methods of creating the required near-surface electronicpotential required for passivation can be developed based on theseconcepts; and that passivation by quantum exclusion may find usefulapplication in a larger class of semiconductor devices and applicationsthan the above-cited examples of solar cells, photodetectors, andback-illuminated solid-state imaging devices.

While the preferred implementation is passivation of silicon surfacesusing MBE growth of doped silicon layers, it is further stipulated thatvarious engineered materials may be designed and fabricated to implementsurface passivation by quantum exclusion in various materials systems,including silicon, alloys containing silicon germanium, and a variety ofIII-V and II-VI semiconductor materials, all of which can be grown anddoped with nanometer-scale precision using the methods of molecular beamepitaxy. Other materials systems and fabrication technologies (such asorganic semiconductors) may also be amenable to the methods and conceptsapplied here.

The ideas and methods presented here can be generalized to encompassmany more device structures and technologies. Epitaxial growthtechnology, together with the theory and concepts of surface passivationby quantum exclusion, are readily extendable to more complicatedstructures and functions, especially with respect to two and threedimensional patterned structures. The ability to fabricate semiconductordopant profiles with nearly atomic-scale precision enables themanipulation of quantum mechanical states and quantum transport ofelectrons and holes. These technologies can thus be applied in thedesign, modification, and development of many conceivable devicestructures, seeking either optimal performance or reduced dimensions ofexisting devices (such as the transistors used as building blocks ofintegrated circuits) or in developing novel devices and structures thatrequire improved surfaces for their practical realization.

The remainder of this disclosure focuses on multilayer passivation ofsilicon, which is a particular instantiation of surface passivation byquantum exclusion that is illustrative of the principles, methods, andadvantages of the inventive technology.

Nature of Delta-Doping as Taught by JPL

To introduce multilayer passivation of silicon, and to provide a basisfor comparison with the prior art, we begin with a description of thenature of surface passivation by delta-doping as taught by JPL, as wellas the problems recently identified with this technology. As applied tooptical detectors, the essential principle of delta-doping is to replacethe thick p+ substrate of a front-illuminated detector with an ultrathinp+ layer that reproduces (in essence) the electric field and potentialbarrier formed by the p⁺p junction of the original substrate/epilayerinterface. Because the ultrathin delta-doped layer is essentiallytransparent (in a qualified sense), back-illuminated, delta-dopeddetectors exhibit extremely high quantum efficiency over the entireelectromagnetic spectrum accessible to silicon (from soft x-rays throughthe near infrared). The essence of the problem of back illumination lieswith problem of surface passivation and stability; in particular, it isessential that chemical and physical changes to the passivated surfacedo not affect detector performance.

As described hereinabove, JPL's delta-doping technology is the bestsurface passivation technology of any in the prior art. JPL's patentsand publications teach that the delta-doped layer should be situatedapproximately 1-2 nm from the Si—SiO₂ interface in order to achieve thebest performance of back-illuminated silicon detectors. Even though thedelta-doped layer taught by JPL is only 2.5 nm thick (equivalent toabout 10 atomic monolayers in the silicon crystal), the sheet density ofdopant atoms in JPL's delta-doped layers is approximately 2×10¹⁴ cm⁻². Adopant density this high should create a highly conductive surface,because the sheet density of dopants is almost two orders of magnitudelarger than the surface charge densities normally present in nativeoxides of silicon.

Low Sheet Densities of Holes in Delta-Doped Surfaces

Recent measurements of surface sheet density (a measure of conductivity)of delta-doped surfaces at JPL show that a near-surface delta-dopedlayer exhibits sheet densities two orders of magnitude lower thanexpected, whereas the sheet density of deep delta-doped layers is withinthe expected range (see Table 1). Profiles of the surface by secondaryion mass spectrometry (SIMS) indicate the delta-doped surface contains asheet density of dopant atoms close to the design value of 2×10¹⁴ cm⁻².This presents a problem for at least two reasons: first, becauseconductivity is an essential function of the substrate that should bereproduced by the surface passivation layer; and second, the low surfaceconductivity indicates that the delta-doped is less robust thanpreviously thought. The low sheet density of delta-doped surfacestherefore demonstrates a need for an improved surface passivationtechnology and provides a basis for evaluating the inventive technology.

Table 1 provides a comparison of delta-doped vs. multilayer-dopedsurfaces based on sheet number (a measure of surface conductivity. Thisrepresents data from MBE-grown layers on ultrahigh purity siliconsubstrates, in order to ensure that the conductivity measurementsaccurately reflect the mobile charge created by doping in the surfacepassivation layer.

TABLE 1 Sheet number Technology Structure (×10¹⁴ cm⁻²) Delta-dopedsurface Shallow delta-layer 0.05 Deep delta-layer* 0.9 Multilayer dopedsurface Two layer multilayer 1.0 Four layer multilayer 4.0 *Note thatdata for the deep delta-layer are presented for comparison purposesonly. Delta-layers buried deep under the surface are not suitable forsurface passivation of imaging detectors, because too much signal islost in the surface region.Dopant Compensation, Chemical Mechanisms, and Models: an Approach toEvaluate Multilayer Doping for Surface Passivation

The discrepancy between the sheet densities of holes and dopant atomsdescribed above indicates that proximity to the surface is somehowcompensating the delta-doped layer. There are at least two possiblereasons for this discrepancy. Either the great majority of dopant atomsin the layer nearest the surface are electrically inactive, or the greatmajority of holes are being immobilized by the surface. Chemicalmechanisms exist that would account for either or both of thesepossibilities; furthermore, these mechanisms involve hydrogen, which iswell-known to be ubiquitous in silicon oxides. Deactivation of surfacedopant atoms such as Boron by subsurface hydrogen is well-known in theart. Injection and trapping of holes in surface oxides is alsowell-known, and is the subject of a relatively large literature. It hasrecently been determined that immobilization of holes in oxides can bean ionic rather than an electrical process, as the injection of holesinto the oxide causes the release of hydrogen from oxygen vacancies,creating both a type of defect known as an E center and also causing thehydrogen to enter into a stable bond with a bridging oxygen atom, thuscreating a fixed, positive charge in the oxide. Bothmechanisms—deactivation vs. immobilization—are therefore associated withhydrogen on or near the surface. Thus the conductivity data show thatimprovements over delta-doping are necessary and provide a quantitativebasis for comparison with models, while the chemical mechanisms offer ameaningful starting point for modeling the surfaces of delta-doped andmultilayer doped silicon. Together, models and data provide a means forcomparing the inventive technology with the prior art, and show thatmultilayer doping provides significant advantages over the prior art.

Modeling the Quantum Mechanical Behavior of Surfaces

Quantum mechanical models of the surface, together with new MBE growthsand characterization data, provide new insights into surface passivationby delta-doping, and illuminate some problems with delta-doping andother state-of-the-art passivation technologies. Calculations andexperiments on improved MBE-grown structures demonstrate the practicalapplication and advantages achieved by using the concepts of surfacepassivation by quantum exclusion to design new device structures andmethods.

The principles of quantum mechanics and semiconductor band theory arenecessary to model the behavior of majority and minority carriers inmesoscale semiconductor structures, thus providing the essentialconnection between composition, structure, and electrical behavior.

Calculations of near-surface properties of MBE-grown layers areessential to illustrate the concepts and applications of surfacepassivation by quantum exclusion. In order to connect theory withexperiment, several approximations are required. These approximationsare essential to making the problem tractable, so that the results ofmodel calculations should be taken as descriptive rather thanquantitative predictions. An effort has been made to make use ofaccepted models and to incorporate as much detail and knowledge ofmaterials as is practical; nevertheless, devices and methods describedhere are to be evaluated based on characterization and performance data,and do not stand or fall based on accuracy of the models.

Because of the relationship between nanometer-scale doping profiles, theelectronic potential of doped semiconductors, and wave properties ofelectrons and holes at nanometer length scales, a theoretical analysisof MBE-grown passivation layers requires quantum mechanical models todescribe the behavior of both electrons and holes in near the Si—SiO₂interface. Here we use self-consistent solutions of the Schröodinger andPoisson equations to model the near-surface band structure; theconduction bands, including the L-point (including the splitting oftransverse and longitudinal modes), X-point, and Gamma-point minima, aremodeled using the effective mass approximation; for the valence bands,an eight-band k·p model is used to incorporate band-coupling effects.

Multilayer Passivation of Silicon Surfaces

Multilayer doping is implemented by growing multiple delta-doped layerson a silicon surface, in which the separation between adjacent layers issmall enough to allow quantum mechanical coupling between layers.Quantum mechanical coupling maintains the high quantum efficiency ofdelta-doping, while multilayer doping increases the surface conductivityby two orders of magnitude compared to delta-doping and provides greaterisolation between the surface and bulk regions. Conductivitymeasurements of MBE-grown layers demonstrate methods and devices bywhich the surface conductivity can be increased by two orders ofmagnitude while achieving, and possibly improving, the isolation ofsurface from bulk silicon that is necessary for effective surfacepassivation.

Modeling Multilayer Doping and Comparing with Delta-Doping

For the purposes of modeling, immobilization of holes at the surfacewill create a surface dipole layer, as charged dopant atoms arephysically separated from charge at the surface by the thickness of thesilicon cap layer. The dipole layer creates an electric field that tendsto confine holes in the semiconductor and electrons at the surface;however, the dipole layer is so narrow that quantum confinement greatlyincreases the ground state energy of electrons confined at the surface,to the point that most of the states are coupled to conduction bandstates in the bulk of the detector. In contrast, deactivation of dopantatoms will effectively neutralize them, thus removing them from themodel as far as calculations of potential are concerned. Therefore, tospan these possibilities, two cases are considered: First,immobilization of charge and the creation of a strong surface dipole,and second, neutralization of dopants and a reduction of the dopantdensity in the layer closest to the surface.

Division into Regions

The plots show electric field and potential energy (FIG. 4, FIG. 5 andFIG. 6), hole concentration (FIG. 7 and FIG. 8), electron states (FIG. 9through FIG. 14), and robustness (FIG. 15 through FIG. 20). The plotsillustrate the principles of the inventive technology by separating themultilayer-doped surface into three regions, as follows:

Chemical Interface

A surface region is bounded by the Si—SiO₂ interface on one side, andthe first delta-doped layer on the other. The chemistry of the Si—SiO₂interface dominates this region. The first doped layer should be closeto the Si—SiO₂ interface in order that this region be subject to quantumconfinement effects in calculated energy states of minority carriers.Quantum confinement in this region helps to minimize trapping ofminority carriers. Trapping of holes in the oxide creates fixed positivecharge and a surface dipole region between the Si—SiO₂ interface and thefirst doped layer. Charge separation in the dipole region creates astrong surface field. Hydrogen generated in the surface can deactivatedopants in the doped layer nearest the surface.

Multilayer

The multilayer region is a new region. Whereas delta-doping representsan abrupt boundary between the chemical and physical interfaces, themultilayer region interposes a region of finite width, with propertiesthat can be controlled by design. In one embodiment, by growing severaldelta-layers instead of one, a “multilayer” of coupled quantum wells iscreated. The separation between layers preferably is narrow, so that thequantum wells are coupled. If the separation between layers is too largecarriers could get trapped in the individual wells, and the quantumefficiency would be low. The dopant sheet densities preferably are highin order to get good isolation between the bulk and the wafer surface. Ahigh barrier provides better suppression of tunneling, and is morerobust against dynamic surface charging, damage and other environmentaleffects. High dopant sheet densities also provide high electricalconductivity, which is lacking in delta-doped surfaces.

Physical Interface

This interface defines the electronic surface of the detector; it is thebeginning of the original detector material that existed prior to MBEgrowth, and is comprised of high purity silicon. This is wherephotogenerated minority carriers need to go in order to be detected, andonce they are there, the multilayer region needs to provide an excellenttunnel barrier to prevent their coming back. The electric fieldextending into this region from the multilayer region and the height andwidth of the potential barrier created by the multilayer region are keyparameters in determining the effectiveness of surface passivation. Theelectric field and potential barrier created by delta-doping are muchlower than expected, based on inferences from the conductivity data.Multilayer doping is far superior to delta-doping and to any other priorart by the various criteria illustrated by the models and confirmed bythe data.

Fabrication Methods of Implement Multilayer Passivation

Because multilayer doping requires the growth of a plurality of dopedlayers on the back surface of a silicon detector, the methods previouslydeveloped for thinning and delta-doping silicon detectors can be used toprepare the surface and grow the first doped layer. Subsequent dopedlayers are formed by an iterative growth process, in order to form thedesired number of doped layers in the multilayer region. While themultilayers formed by this method are generally taken to be periodic,the inventive technology of multilayer doping for detector passivationdoes not require that all layers be formed identically.

In one preferred embodiment, the preferred method for multilayer dopingincludes the following process steps. Note that some steps may be added,altered, eliminated, or performed in a different sequence, depending onspecific requirements for different detector designs.

-   -   1. Supporting the detector prior to thinning the wafer by a        frame-thinning process in which thinning leaves a thick frame to        support the thinned region, or by bonding the detector to a        mechanical support prior to thinning in order to thin the entire        device.    -   2. Cleaning the surface to be thinned, for example, using a        standard cleaning process for silicon wafers, such as the RCA        cleaning process.    -   3. Thinning the detector, for example, by a series of steps        including chemical-mechanical polishing, chemical etching with a        heated KOH solution, chemical etching with a mixture of        hydrofluoric and acetic acids, and etching with a solution of        KMnO₄.    -   4. Cleaning the back surface of the thinned detector, for        example, by another RCA cleaning step, followed by a UV ozone        cleaning process.    -   5. Hydrogen passivation of the surface, for example, by placing        the detector on a spinner in a nitrogen environment, and        exposing the surface to a sequence of chemicals while spinning        including ethanol, an HF:ethanol mixture, and ethanol again.    -   6. Loading the device into a vacuum chamber and pumping to        ultrahigh vacuum pressures.    -   7. Transferring the device under vacuum into the MBE growth        chamber.    -   8. Annealing the device at low temperature to remove volatile        chemicals from the surface, for example, by heating to 150° C.        for at least 10 minutes.    -   9. Heating to a temperature of at least 380° C. and not more        than 450° C.    -   10. Growth of a silicon layer as a buffer layer to produce an        atomically clean silicon surface.    -   11. Stop silicon growth.    -   12. Optionally cool the device to a lower temperature, for        example, to a temperature between 250° C. and 300° C. for growth        of n-type multilayers.    -   13. Perform iterative growth of a plurality of delta-layers: For        each delta-layer in the multilayer, deposit dopant atoms until        the desired dopant density is reached, stop the flux of dopant        atoms, and grow a desired thickness of silicon over the        delta-layer. For example, a dopant density of 2×10¹⁴ cm⁻² and a        silicon layer thickness between 1 and 2 nm may be used for each        delta-layer. It is not required that each layer be identical to        the previous layer.    -   14. Cool the device gradually, and remove from the MBE chamber.    -   15. Optional steps for oxide formation and antireflection        coating, as necessary for specific applications.    -   16. At this point the passivation by multilayer doping is        complete, and additional steps for packaging may be performed as        needed.

FIG. 21 is a schematic, not to scale, diagram that illustrates a crosssection of a wafer 2100 having multilayer doping according to principlesof the invention. In this example, a silicon semiconductor wafer isdescribed, having deliberately provided semiconductor devices thereon.In FIG. 21, semiconductor devices (such as a CCD array in oneembodiment) are provided on the free surface of the layer 2160 of thewafer shown at the bottom of FIG. 21. Illumination represented by arrows2105 is expected to impinge on the wafer from the back surface side(opposite to the surface where the semiconductor devices are provided).Layer 2150 of the wafer represents the remaining bulk material with asurface present after an optional thinning process is applied to theback side of wafer 2100. Layers 2115, 2125, 2135, and 2145 and layers2120, 2130 and 2140 are grown on the thinned wafer. In the exampleillustrated, layers 2115, 2125, 2135 and 2145, presented in partiallydarkened fill, represent four doped layers that include a density of adeliberately added dopant species (such as a p-type dopant such asboron, or an n-type dopant such as phosphorus or antimony). The wafer2100 need not have exactly four doped layers, but in general a pluralityM of doped layers, where M is an integer greater than 1. The dopantsheet densities in the M doped layers need not be the same, but inprinciple can be selected to be the same sheet densities or to bedifferent sheet densities. Interleaved between layers 2115, 2125, 2135and 2145 are M−1 (here with M=4, M−1=3) layers 2120, 2130 and 2140 thatare not deliberately doped (also referred to as “undoped layers”), forexample, layers that are substantially silicon having no deliberatelyadded dopant. Structures with M=2, M=3 and M=4 have been demonstrated.Layer 2110 is a final semiconductor layer of the wafer provided bygrowth after all of layers 2115 through 2145 are grown, so that anynecessary electrical contacts or optical antireflection layers can beprovided on the back surface of wafer 2100. Layer 2100 may be doped asdesired or as may be convenient. In general, the plurality of M dopedlayers 2115, 2125, 2135 and 2145 can be as thin as a single layer ofsilicon (approximately 2.5 Angstroms) and can be doped at sheetdensities up to approximately 2×10¹⁴ cm⁻² dopant atoms. One way tomeasure dopant density is sheet density, which is measured in dopantatoms per square cm. The M−1 layers 2120, 2130 and 2140 that are notdeliberately doped can have thicknesses in the range of 5 Angstroms to40 Angstroms, and are preferably grown with thicknesses in the range of10 Angstroms to 30 Angstroms.

Because some crystal growth methods are kinetically controlled and arenot processes that attain a thermodynamic equilibrium, it is expectedthat it may be possible in a different (second) embodiment to grow theplurality M of doped layers without providing M−1 interleaved undopedlayers between adjacent doped layers. This might be accomplished, forexample, by allowing a first flux of dopant to impinge the growthsurface for a first duration of time (thereby providing less than acomplete monolayer of dopant), allowing a flux of silicon to impinge thegrowth surface for a second duration of time (thereby completing acrystalline monolayer), and then growing another monolayer by using asecond dopant flux and a second silicon flux for additional durations oftime, respectively. By changing the flux and the time of impingement,one may expect to grow a sequence of layers having a series of desireddopant sheet densities.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, or publication identified in thespecification is hereby incorporated by reference herein in itsentirety. Any material, or portion thereof, that is said to beincorporated by reference herein, but which conflicts with existingdefinitions, statements, or other disclosure material explicitly setforth herein is only incorporated to the extent that no conflict arisesbetween that incorporated material and the present disclosure material.In the event of a conflict, the conflict is to be resolved in favor ofthe present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A silicon device, comprising: a silicon wafer bounded by a firstsurface and a second opposite said first surface, said silicon waferhaving a device fabricated on one of said first surface and said secondsurface; said silicon wafer having a doping profile situated adjacent atleast one of said first surface and said second surface, said dopingprofile having a plurality M of doped layers; each of plurality M ofdoped layers having a thickness of less than 10 Angstroms, and a dopantsheet density at least 10¹⁴ cm⁻², where M is an integer greater than 1;said plurality M of doped layers separated from each other by M−1interleaved layers of silicon, at least one of said M−1 interleavedlayers of silicon having a thickness in the range of 10 Angstroms to 30Angstroms; said silicon wafer having at least one of said first surfaceand said second surface electronically passivated irrespective of adensity of defects present on said respective one of first surface andsaid second surface.
 2. The silicon device of claim 1, wherein M is atleast 3, and said plurality M of doped layers are separated by M−1interleaved layers of silicon, at least two of said M−1 interleavedlayers of silicon having substantially equal thicknesses.
 3. The silicondevice of claim 1, wherein M is at least 3, and said plurality M ofdoped layers are separated by M−1 interleaved layers of silicon, atleast two of said M−1 interleaved layers of silicon having unequalthicknesses.
 4. The silicon device of claim 1, wherein at least one ofsaid M−1 interleaved layers of silicon has a dopant sheet density ofless than 10¹³ cm⁻².
 5. The silicon device of claim 1, wherein a dopantgradient of at least one decade per nm exists between one of saidplurality M of doped layers and an adjacent one of said M−1 interleavedlayers of silicon.
 6. A silicon device, comprising: a silicon waferbounded by a first surface and a second opposite said first surface,said silicon wafer having a device fabricated on one of said firstsurface and said second surface; said silicon wafer having a dopingprofile situated adjacent at least one of said first surface and saidsecond surface, said doping profile having a plurality M of dopedlayers; each of plurality M of doped layers having a thickness of lessthan 40 Angstroms, a dopant sheet density at least 10¹⁴ cm⁻² and adopant gradient of at least one decade per nm, where M is an integergreater than 1; said silicon wafer having at least one of said firstsurface and said second surface electronically passivated irrespectiveof a density of defects present on said respective one of first surfaceand said second surface.